Adrenocortical responsiveness to ether vapor stress was measured in young adult (4–6 months) and aged (22–32 months) rats subjected to acute and chronic treatments with dexamethasone. Daily administration of 25 µg dexamethasone/100 g body weight for 28 days markedly reduced adrenocortical responsiveness to stress in all groups. Both young and aged groups returned to normal stress responsiveness after cessation of the treatment. Injection of 5 µg dexamethasone/100 g body weight for 14 days produced greater adrenocortical inhibition than did 1 µg treatment in all groups. Aged groups of both sexes showed less suppression of stress activation of the adrenal cortex on days 1, 4, 8, and 14 of both the 1 and 5 µg/100 g body weight treatment regimes. Adrenocortical responsiveness to stress at 4, 7, and 15 min after animal disturbance was also less inhibited in aged male and female rats receiving a single 5 µg/100 g body weight injection of dexamethasone than in similar groups of young animals. These data indicate that the aged rat has a substantial decrease in the sensitivity of its adrenocortical control mechanism to feedback inhibition, which may play a substantial role in the homeostatic alterations recognized to occur with increasing age.